“Moreover, one cannot know how many secular Jews become observant and what percentages go to different groups – and whether they stay with a certain group or move on to another stream (this can happen when they initially become very machmir and then stabilize in the middle).”
Its difficult. But there are demographic studies all the time. They give a sense — i think. The MO study did a decent job.
The other way — of course — is you do a small study and extrapolate from the numbers.
A poster above said that “MO are leaving in droves”. I dont know how many MOS are in a “drove” but that aside — I am curious about the “drop out” numbers and even more curious about what drop out means.